Current techniques for positioning include the following:                Cellular: Global Positioning System (GPS), Assisted GPS (A-GPS), Time of Arrival (TOA), Radio Fingerprinting, Cell Global Identity, and Enhanced Cell ID.        WiFi: Radio Fingerprinting, WiFi TOA, and Wireless Access Point Wardriving.        GPS: Differential GPS (DGPS), Wide Area Augmentation System (WAAS), A-GPS, and Laser Augmentation.        
GPS is a space-based satellite constellation that provides constant, global geopositioning to end users. A user's GPS device processes signals from three or more GPS satellites, and by way of trilateration provides latitude and longitude data. GPS may be global, but it is not ubiquitous because foliage, building edifices, and bridges block GPS signals. The absence of GPS signals in places such as the urban core is problematic because high value applications for reliable positioning abound, yet no reliable solution exists. The urban core in major cities is also known as the “urban canyon,” because skyscrapers on both sides of the streets block GPS signals. Another common problem is multipath distortion in which signals bounce off many surfaces before reaching the user, resulting in vastly inaccurate positioning data.
In addition to providing navigation within the urban core, GPS is widely used to aid navigation worldwide. For example, GPS is a routine component of aviation navigation at all levels including the next generation air traffic control system referred to as Automatic Dependent Surveillance-Broadcast (ADS-B). See, <<http://www.faa.gov/news/fact_sheets/news_story.cfm?newsId=8145>> (last accessed Sep. 25, 2008). ADS-B is anchored on GPS derived position and velocity information. ADS-B takes advantage of GPS accuracy, reliability and global coverage to improve navigation performance for improved safety and efficiency. Because of satellite-based navigation (SATNAV) vulnerabilities, however, it is widely recognized that ADS-B must have a networked system-wide backup.
GPS was originally designed as a military system and has had recent broad adoption for civil and civilian uses due to its inherent accuracy and a strong history of reliable performance. Several other entities, including the European Union, Russia, India, China, and Japan have satellite-based Position, Navigation, and Timing. Collectively, the use of satellite-based navigation services is referred to a Global Navigation Satellite Systems (GNSS). GPS, however, remains the de facto system of choice, primarily because of its maturity, with over fifteen years of reliable free service. GPS acceptance has also been enhanced by open interface control documentation (ICD), which allows receiver manufacturers to confidently design systems against a reliable standard.
These satellite systems provide a very low cost-of-ownership for many applications including, for example, civil aviation management compared to traditional radar methods and have the added advantage of global coverage for international flights. However, because they are satellite-based, there is a danger that either natural or human threats to these systems could make them vulnerable to outages. Thus, there is considerable work going into a search for an aviation back-up system to provide navigation information in the event of GPS outage. See, for example, <<http://www.jpdo.gov/library/InformationPapers/ADS-B_Backup_Report_PDA.pdf>> (last accessed Sep. 26, 2008).